In the feeding of powdered, granular, flake or the like material from a supply hopper onto a conveyor or into containers, a common problem is "bridging" of the material in the hopper, whereby self-supporting arch of the material forms between opposite walls of the hopper. Many devices have been proposed for preventing or correcting such bridging, such as means for vibrating the hopper walls, pulsating devices, and internal mechanical agitators, all of which have disadvantages, since most of such devices must be built as an integral part of the hopper when it is originally manufactured, and cannot be readily applied as retrofit equipment. Devices of the vibrating type which can be attached to the outside of the hopper wall are often not effective with large hoppers because of the large mass involved, and are not effective at all with certain types of materials. Mechanical agitating devices have a high original cost and require considerable maintenance, and when the device is being repaired, the hopper is out of service.
In some cases blasts of air applied through the wall of the hopper at regular intervals has been found effective to prevent bridging, however in large hoppers or with certain types of heavy material, for such an air blast to be effective,the air pressure must be higher than can be safely used, and in situations where an air blast must be used more or less continuously, the expense of producing the compressed air can be a significant addition to the cost of the operation.